1. Field Of The Invention
The invention relates to a surface-treated abrasive grain on the basis of silicon carbide and its use in resin-bonded or ceramic-bonded grinding wheels.
2. Background Art
Silicon carbide is one of the most important abrasives and is used especially for grinding and cutting off hard and brittle materials. It is used both for loose abrasives and for abrasives on a support and abrasive wheels such as grinding and cutting-off wheels. The grinding performance and especially the wear ratio or service life of the abrasive wheels, thus, depends not only on the properties of the abrasive grain itself but also to a considerable extent on the type and strength of the binding of the abrasive grain in the binder. Especially with resin-bonded abrasives, the adhesion of the silicon carbide grain in the binder generally is the factor determining performance and wear. Therefore, numerous attempts have been made to improve the binding by a suitable treatment of the very smooth surface of the silicon carbide.
A frequently used method is the production of a ceramic coating which coats the grain like a glaze. U.S. Pat. No. 1,910,444, for example, describes such a process. If a slightly meltable material in a particle size that is considerably less than the abrasive grain is added to the coating, a roughened coating is obtained, as described, for example, in U.S. Pat. No. 2,527,044.
But considerable drawbacks are inherent in all of such processes. They require several process steps, including a firing or sintering process and, therefore, are time- and energy-consuming. In addition, agglomerates are formed by caking of the individual grains. Such agglomerates must be removed by an additional screening process or must be mechanically destroyed, and the just applied coating is again partially lost. Below a certain grain size the agglomerate formation is so predominant that the process can no longer be used.
In ceramic-bonded abrasive wheels the use of ceramically coated abrasive grains is generally not advisable, since during firing of the abrasive wheels at about 1150.degree. to 1400.degree. C. the binder of the coating again melts and, together with the applied fine grain (e.g., Fe.sub.2 O.sub.3), penetrates into the ceramic mass and can initiate disturbing reactions there which, for example lead to pore formation or color deviation. Further, when the abrasive wheels are used, especially in dry grinding, the binder can soften by the development of heat and cause the grain to break off.